With reference to FIG. 1, a ducted fan gas turbine engine is generally indicated at 10 and has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, an intermediate pressure turbine 17, a low-pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.
During operation, air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first (main) air flow A into the intermediate pressure compressor 13 and a second (bypass) air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 13 compresses the air flow A directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.
It is known to bleed gas through bleed valves from the first air flow A into the bypass duct 22 when the engine is at low power in order to maintain the desired pressure ratio between the Inlet and outlet of the intermediate compressor 13 to avoid engine stalling and/or engine surge. These bleed valves may be binary (i.e. with an open and a closed position) or fully modulated. The fully modulated bleed valves are typically associated with complex feedback control systems and valve actuation mechanisms.
It is also known to source compressed cooling gas from first air flow A before the combustion equipment 15 for feeding to engine systems/components requiring cooling. Often, these engine systems/components require only minimal cooling gas flow at low engine power.
For example, it is known from U.S. Pat. No. 5,261,228 to bleed compressed air from a core duct through a binary bleed valve to the bypass duct and to a clearance control system (at a low rate) in a first mode of operation (with the bleed valve open) and to bleed air from the bypass duct to the clearance control system (at a higher rate) in a second mode of operation (with the bleed valve shut). This arrangement does not allow full modulation of the flow to the clearance control system.
There is a desire to provide a bleed arrangement that allows bleeding of gas from the main gas flow path at a variable flow rate to a number of different engine components/systems without the requirement of a complex actuation mechanism.